The present invention relates to an improvement in the steel refining method which makes use of a slag.
Refining of a steel, particularly desulfurization of the steels by a basic slag, is carried out through the following interfacial reaction between the slag and the molten steel. EQU 3CaO+3S+2Al.fwdarw.3CaS+Al.sub.2 O.sub.3
In order to attain a fast desulfurization, it is necessary that the following three conditions are met.
(1) The above-mentioned reaction takes place and proceeds quickly. PA1 (2) The S contained in the molten steel quickly transfers to the interface. PA1 (3) The formed CaS diffuses into the slag from the interface. PA1 (a) The transfer of the S in the molten steel as mentioned in (2) above is promoted in the intial stage thanks to the bubbling. PA1 (b) However, in the initial stage, the transfer of CaS in the slag mentioned in (3) above takes place only at a low speed so that the reaction mentioned in (1) above also proceeds at a corresponding small rate. PA1 (c) In the final stage, the transfer of S in the molten steel is rate-determining.
Hitherto, in the steel refining making use of a slag, an inert gas such as argon gas is blown into the vessel containing the molten steel from the bottom of the vessel through a porous plug to stir the molten steel through bubbling thereby to promote the refining.
The present inventors have made an intense study on the mechanism of the above desulfurization, in order to eatablish a technology for attaining a quicker steel refining, and have discovered the following facts.
These facts suggest that the refining speed can be increased by effecting a stronger stirring by bubbling. It is, however, not allowed to strengthen the bubbling unlimitedly, because the vessel containing the molten steel usually has the form of a ladle, and the height of freeboard, i.e., the wall above the molten metal surface of this vessel is as small as 30 cm and does not exceed 50 cm at the greatest, and therefore, a too vigorous bubbling tends to cause an overflow of the slag or molten steel beyond the top of the wall of the vessel. Needless to say, it is not preferred from the view point of effective volume of the vessel to preserve a large freeboard.
A further study proved that the conventional stirring by bubbling suffers from the following problem. Namely, this conventional method inevitably causes a local dulling of the movement or flow of the slag. As a result, the interfacial reaction and the movement of CaS in the slag are impaired in these regions where the movement of the slag is dull. In addition, this conventional method cannot provide sufficient stirring effect uniformly over all portions of the molten steel.